DEX improvements

[whoabuddy]

Noting some things we see along the way that can be improved, especially in the context of them being deployed in the DAO.

1. constant token-supply can call get-total-supply in token contract
2. constants VIRTUAL_STX_VALUE and COMPLETE_FEE could be computed inline (20% and 2%)
3. constant BURN_ADDRESS can be removed in favor of calling ft-burn? in the token contract
4. in initialization, token-balance could be set by calling get-balance in the token contract for (as-contract tx-sender)

These optimizations take advantage of the token contract being deployed first so we can create functions that call it versus using a sip-010 trait and matching value.